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Chris
I can’t figure out why my 1963 Mercury Monterey with a 390 motor is still backing firing while it idols ONLY! My mechanic checked the timing and set it, but still back fires! The carburetor is older, but would that cause a backfire? Also, we added new plugs, wires and distributor cap but still backfires.
I’m at LOSS! Any suggestions are welcomed!
JoshMake sure plug wires aren’t crossed, have seen that a couple times. Some cars will still start up and run and have a backfire or pop every few seconds because the spark from the plug isn’t timed with the cylinder stroke because of the wires being crossed.
Just throwing ideas out there, there are plenty of other possible causes but try the easy stuff first.
SteveBroken intake spring or hanging intake valve.
El BrujoTiming chain. Distributor being off a tooth or two. I’ve seen 2 plug wires crossed cause it. Could be a carb issue too. But your plugs would have told you that it was running too rich.
ChrisCompression and leak down… valve seals, valves, cracked head, definitely find a mechanic that specials in classic cars.
MikeIs the engine been rebuilt? It’s possible that you have burnt a valve due to unleaded fuel.
Also, it maybe as simple as it’s running too rich and off idle, it has enough air to burn correctly.
JordanCould also be floating valves…Ike others have said do a leak down test and see what your cylinders look like. I’d the carb is running lean or rich, it could also cause it.. set your floats – check for proper tuning (lots of videos on YouTube) set idle, set idle mix to best idle, reset idle rpm, set idle mix screws to best idle again. Check to see if your backfire ceases.
You should post a video not pictures
DaleDistributor advance might not be set, or the cap might have moisture or carbon tracking. Carbon on the valves as a couple of ideas.
KennethIt doesn’t make sense, but back in the day, the routing of the wires would make a car backfire at idle.Timing and firing order were fine, but you had to put, I believe, it was 5 over the top of 7 !
DominguitoCheck the Airfilter Element, maybe its clogged or damage already, in short Rostricted Air,
ToddAt idle only I suspect camshaft timing, valve or guide issue or a problem in the carburetor idle circuit.
As stated identify if it’s back firing into the intake, or is it popping in the exhaust.
A quick test for valve timing or valve sealing issues is to perform a check of the intake vacuum.
If it’s jumping around, then compression is getting into the intake. Low vacuum look for intake leaks.
Compression a d leakage tests are a good idea to rule out or confirm mechanical issues.
RonVerify firing order and routing of the wires. Put a dwell meter on it and adjust points properly, do timing last.
ScottIf your timing is correct then it’s in the idle mixture. Check for vacuum leaks with carb cleaner. You need to find someone in your area that really knows how to work on carbs, it’s not easy. What’s base timing set to?
Ed DegelsmithMake sure the jets on the carb are clean. I had a motorcycle that did the same thing and when I had the carburetor off I decided to clean it. Then the backfire popping sound disappeared.
EricEasy. It’s running rich. Too much fuel. When at WOT, it has enough air to burn the fuel. I wouldn’t check compression until you figure out the fueling situation first.
ChristianTake the air cleaner off, start it, take a pair of pliers and smack the carb body few times. Then open the throttle plates while running and cover the carbs intake with your hand or your hand with a rag a few times, don’t let it suck the rag into the carb.
You’re choking it without killing it. See if it idles after this. Sounds like some things inside the carb are gunked up and sticking.
MichaelI would start by letting it idle and spraying some fluid around the carburetor gasket and the intake gaskets… if it changes the idle, that is more than likely the culprit… try to snug the carb bolts down first.
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